I was pricing out a group for my hypothetical bike and noticed something... the cost is exponential by level. These are the prices for the basic groups at http://www.ribblecycles.co.uk
I admit faking this a little. Without the Ultegra Di2 there is a "missing link" at that price level. They don't presently have Dura Ace Di2 listed as a group, that's why it's missing. Campagnolo follows a similar trend though not as cleanly. They don't offer SRAM.